Notch signaling regulates multiple aspects of hematopoietic development. Notch receptors are aconserved family that influence cell fates, survival, proliferation, and borderformation in multi-cellular organisms.This competing renewal represents our long-term plan to understand the function of Notch signaling in hematopoietic development and function. We and others have providedevidence that Notch regulates T cell commitment from a multi-potential progenitor and regulates multiple aspects of intrathymic T cell development, The proposed studies focus on the function of Notch signaling in T cell commitment and early thymocyte development, In Specific Aim 1, we will identify the bone marrow, intra-thymic and extra-thymic progenitors upon which Notch acts to specify T cell commitment.In Specific Aim 2, we will determine the function of Notch signaling and identify the mechanism by which Notch influences multiple events during early thymocytedevelopment. In Aim 3, we will determine the mechanism that downregulatesNotch signaling at the DN3-DN4 transition. Our studies will utilize a combination of murine models, including a novel loss-of-functionmodel created by us, organ and stromal cell cultures, and biochemical approaches.In particular, we seek to understand the function of Notch signaling at the pre-TCR checkpoint, the downstream signals by which Notch regulates intrathymic T cell development, and the mechanisms regulating Notchl transcription. Together, these studies will lead to an improved understanding of lymphocyte developmentand function, and in doing so, will provide insights into modulating the immune system for therapeutic advances.